1. Field of the Invention
The present invention refers to a heat exchanger device, also called recuperator, which advantageously is intended for use with a gas turbine for stationary use in a small-scale power station or for mobile use in a vehicle. The invention also refers to a method for manufacturing the heat exchanger.
2. Background Art
A heat exchanger of the above-mentioned type is used, for example, in small-scale power stations, for mobile use or in a reserve power station. In at least these applications, it is of great importance that the recuperator be designed in such a way that it be as effective as possible, while at the same time being light-weight, and with dimensions that are minimized or at least diminished. Recuperators conventionally have a number of plates made from a very thin plate material with a thickness of normally about 0.1 mm. The plates exhibit, in a known manner, corrugations and are stabilized toward each other in a wave shaped pattern. As a consequence, flow channels for a heat absorbing medium and flow channels for a heat-emitting medium are formed between the corrugations. In the case where a gas turbine is used, the heat-emitting medium is combustion gas from the gas turbine and the heat-absorbing medium is normally air. It is of great importance for the efficiency of the recuperator as to how the corrugations are stabilized against each other, what angle is formed between the corrugations, and the size of the flow channels. All of the mentioned factors affect the flow characteristics in the recuperator, and the flow characteristics are a big part of optimizing the heat exchange between the heat absorbing medium and the heat-emitting medium.
According to one known method for manufacturing a flat heat exchanger, the plates are soldered together into a cell, with the aid of vacuum soldering. This method operates satisfactorily when manufacturing stationary flat heat exchangers where the thickness of the plates is about 0.2–0.4 mm. In order to manufacture recuperators for mobile use or small-scale power stations, even thinner plates are necessary having thicknesses of about 0.1 mm. The recuperator intended for small-scale power stations has plates that consequently are very thin, and as a result problems occur when using previously known vacuum soldering techniques for the joining of the plates. The proportionately high quantity of heat that is supplied to the material when using vacuum soldering technique may lead to a localized, thermally conditioned structural collapse of a number of corrugations in the thin plates, whereby the flow channels are narrowed or blocked and where the function of the recuperator no longer can be ensured.
It is also previously known to form cells by welding the plates in pairs with spacing elements lying in-between the plates with the aid of TIG-welding apparatus, but problems will occur here also. One of the problems when welding thin plates with a TIG-welding apparatus is that the cell will buckle due to the fact that the heat distribution is too great.
Even if known systems are well functioning, improvements are possible with respect to obtaining a more compact recuperator. In conventional designs, the plates that form the cells, and thus the recuperator, are formed with specially adapted channel openings in the plates that are supposed to fit to channel openings in other plates and thereby form to and from flow channels. This has resulted in unnecessarily high demands on fit and tolerances and has also made it difficult to manufacture one plate in one process step. In order for the recuperator to function satisfactorily, high demands are put on fit and tightness between the plates and the inlet and outlet channel openings of the recuperator, as well as the formed rows of flow channels in the recuperator. The manufacture of such a recuperator may be expensive and the special demands make the device somewhat inflexible.
A further drawback with know devices of this nature is that the flow channels in the recuperator give a fairly skewed distribution and causes high pressure losses. These effects result in deterioration in the efficiency of the system and obstructs the distribution of the various media.
When using previously known techniques as described above, cells are formed either by welding or soldering of plates in pairs with interstitial or interjacent spacing elements. In this way flow channels are formed between the plates for the heat absorbing medium. When forming the recuperator, the cells are stacked against each other so that flow channels for the heat-emitting medium are formed between the cells. One demand on a well-functioning recuperator is that it be gas tight at least at certain places. The demand for gas tightness puts high demands on the contact surfaces between the cells on at least two opposing edge parts that must be made gas tight. Conventional recuperators have made the end parts gas tight by using bolting arrangements that press the end parts towards each other, and also gas tight cover plates placed over the joints in between the cells, or a gas tight box. The cover plates are thereafter welded to an L-flange and placed around the inlet opening for the heat-emitting medium. In this manner, the flange also serves as a connection flange for the heat-emitting medium. The cover plates are also welded to junction channels placed on the recuperator formed by the cells. In spite of bolting and cover plates, leakage of the heat-emitting medium to the surrounding environment is common when using this construction of the recuperator. When using bolting, a number of bolts are placed perpendicular to the plates and perpendicular to the flow direction of the heat-emitting medium. When using such recuperators, a number of problems occur. One of the problems is the difficulty with the fit in order to get the contact surface between the cells gas tight, which puts high demands on tolerances of the gas tight side parts that are to be fitted in between the cells formed joints, and also demands very clean and flat contact surfaces. A further problem is that the bolts in the bolting arrangement have to be put in the correct place for the construction, and have to be tightened in such a way that a high and even pressure is applied over the gas tight contact surfaces between all the cells. Still a further problem with using bolting is that the bolts are placed in the heat-emitting medium stream flow and interfere with the flow characteristics and the distribution of the medium over the channel inlets. The recuperators mentioned above are consequently difficult, and thus expensive, to manufacture and are also large and heavy in relation to their efficiency.
In order for a recuperator suitable for use in small-scale power stations to be commercially competitive, it is a demand that the recuperator have a small volume compared to the amount of energy used. It is also a demand that the recuperator be simple to manufacture and have a low manufacturing cost.